One of the flow soldering apparatuses performs soldering in that a printed circuit board is first soldered with a primary jet nozzle, and then a secondary jet nozzle adjusts soldering defects such as bridges, icicles, etc. caused through the soldering procedure of the primary jet nozzle (refer to, for example, Japanese Patent Application Laid-Open No. H9-8450). In this type of the flow soldering apparatus, a surface of solder has been oxidized which then produces impurities called dross. As a countermeasure thereto, a surface of a solder layer may be filled with inert gas (refer to, for example, Japanese Patent Application Laid-Open No. H9-47866), or the surface of the solder may be covered with anti-oxidizing oil such as vegetable oil. However, the former incurs increase in cost for using of the gas, which disadvantages a low-cost feature given to the flow soldering. On the other hand, the latter involves a problem of that stink or foreign matters occur due to thermal decomposition of the oil. The flow soldering apparatus may be provided with a dispersion protective-shield for foreign matters, which covers an upper portion where molten solder flowed and dropped from the nozzle strongly collides against a solder surface in a solder tank or flow from another nozzle (refer to, for example, Japanese Patent Application Laid-Open No. 5-305432).
Referring to FIG. 9, the molten solder both flowed from the primary jet nozzle and the secondary jet nozzle in the solder tank is moved downward and met each other between the jet nozzles. The dross tends to build up at the merging point D. Once the dross occurs, it spreads as covering the surface of the solder, and the dross eventually spreads upward over the surface flowed from the primary jet nozzle and the secondary jet nozzle. In order to prevent the upward spread of the dross, the interval between the solder flowed from the primary jet nozzle and the secondary jet nozzle is generally kept at 5 cm or more.
However, the foregoing flow soldering apparatuses have had the following drawbacks. Because the dross spreads upward over the flowed surface of the primary jet nozzle and the secondary jet nozzle, the dross attaches to the solder surface of the circuit board which causes soldering defects such as bridges, icicles, etc. Considering cost and productivity, the above soldering manner may be appropriate for home-use electrical products but cannot be applied to fine-pitch electronic components since the bridges cannot be eliminated completely. For the circuit board with the fine-pitch electronic components which is soldered in the conventional solder tank, operators need to perform visual inspection to solder portions after the circuit board is soldered. In case the bridges are detected, the operators are required to repair the circuit board with an iron, which hampers productivity.
Further, because the interval between the solder flowed from the primary jet nozzle and the secondary jet nozzle has approximately 5 cm, the solder attached to the circuit board cools down while the circuit board moves between the jet nozzles, whereby the performance of the soldering at the secondary jet nozzle becomes deteriorated. Also, for being incapable to reduce the interval between the nozzles, the above-mentioned flow soldering apparatuses have difficulties to be miniaturized.
Furthermore, in case the conveying speed of the circuit board between the primary jet nozzle and the secondary jet nozzle is made faster for not having the solder attached to the circuit board cooled, time in which the circuit board faces these jet nozzles gets shorter, whereby temperature of the solder on the circuit board surface is restrained from rising, causing soldering failure, or defects such as bridges, icicles, etc.
Still further, although a lead-free solder has been used in recent years for an environmental protection, the lead-free solder has a high-melting point (approximately 230° C.) compared to a conventional solder (approximately 183° C.). The lead-free solder is thus more affected by the temperature drop-down on the circuit board than the conventional solder. Accordingly, in order to use the lead-free solder, the interval between the primary jet nozzle and the secondary jet nozzle needs to be further reduced for preventing the solder from dropping-down of the temperature.
The present invention has been made in light of the above problem, and it is an object of the present invention to provide a compact soldering apparatus applicable for a lead-free solder while reducing an occurrence of dross to improve product reliability.